The overall goals of this proposal are a) to define the mechanisms whereby extracorporeal circulation (ECC) activates blood constituents and causes pulmonary and hemostatic dysfunction, and b) to develop effective intervention(s). The exposure of blood to biomaterials comprising the ECC apparatus activates leukocytes, platelets and plasma zymogens, including the complement, contact, coagulation, and fibrinolytic cascades, giving rise to pulmonary and hemostatic dysfunction. Since in vitro studies cannot define the pathogenetic relevance of the multiple complex reactions postulated to produce organ dysfunction, we propose to measure the capacity of pathway-specific, and then molecule-specific inhibitors to prevent activation of blood constituents, and interrupt pulmonary and hemostatic dysfunction in a well-characterized baboon model of ECC. We postulate that the acute damaging effects of ECC on pulmonary and hemostatic function are mediated by serine proteases arising directly from surface activation of zymogens or indirectly from cellular synthesis or secretion. Using the baboon model, we will initially characterize ECC-induced activation of blood elements, pulmonary dysfunction, and hemostatic dysfunction. We will then determine the effects of blocking the putative serine protease pathway(s) on ECC-induced abnormalities by administering a combination of potent and safe protease inhibitors, each exhibiting different specificities. If global inhibition of serine proteases prevents pulmonary and hemostatic dysfunction, the effects of enzyme-specific inhibitors will then be investigated. Alternatively, failure of serine proteases to produce benefit, will direct efforts towards the evaluation of other potential pathways in the production of ECC-mediated organ damage.